Piston ring and method of manufacture

ABSTRACT

A piston ring and a method of manufacturing a piston ring for a piston of a reciprocating internal combustion engine. The piston ring comprises a body having an outer circumferential surface. A tribological coating is formed on the outer circumferential surface of the body. The tribological coating has a dual layer structure and includes a relatively hard base layer and a relatively porous top layer overlying the base layer.

TECHNICAL FIELD

The present disclosure generally relates to a piston ring for a pistonof a reciprocating engine, and more particularly, to a coating for apiston ring.

BACKGROUND

A piston ring is an open-ended ring that fits into an annular grooveformed in an outer circumference of a piston of a reciprocating engine,such as an internal combustion engine. A typical piston is equipped withmultiple piston rings, including a top compression ring, an oil controlring, and a scraper ring. Many piston rings are constructed with alarger relaxed diameter than that of the cylinder in which they will bedisposed. When disposed within a cylinder of the engine, the pistonrings are compressed around the piston due to their intrinsic springforce, which ensures sufficient radial contact between the rings and aninner wall of the cylinder. During engine operation, the piston moves upand down within the cylinder and the radial pressure exerted on thecylinder wall by the piston rings provides a seal around the piston thatisolates the combustion chamber from the crankcase. Gas pressure fromthe combustion chamber may increase the sealing capability of the pistonrings by forcing the rings outward and increasing the radial contactpressure between the piston rings and the cylinder wall.

An effective gas-tight seal between the piston and the inner wall of thecylinder is necessary for efficient engine operation and is the primaryresponsibility of compression-type piston rings. Compression rings arelocated closest to the combustion chamber and help prevent a phenomenonknown as “blow-by,” wherein combustion gases leak from the combustionchamber, past the piston rings, into the crankcase. In addition,compression rings also help control oil consumption by preventing excessoil not needed for lubrication from traveling in the opposite directionfrom the crankcase into the combustion chamber. To obtain an effectiveseal between the combustion chamber and the crankcase, compression ringsmust constantly and fully contact the inner wall of the cylinder.However, due to manufacturing tolerances and the thermal and mechanicalloads imparted on the engine, the shape of the rings may not alwaysmatch that of the cylinder in which they are disposed.

After initial assembly of a new “green” engine, the piston rings may notperfectly conform to the shape of the cylinder in which they aredisposed. In such case, the piston rings must undergo a break-in orrunning-in phase wherein the rings are seated to the cylinder wall bybeing physically worn into the cylinder wall until an effectivegas-tight seal is established therebetween. During this initial break-inperiod, combustion gas blow-by and excess oil consumption by the enginemay occur due to gaps or local variations in the contact pressurebetween the piston rings and the cylinder wall. Accordingly, it isdesirable to reduce the duration of the initial break-in phase so thatthe engine reaches its optimum operating efficiency as quickly aspossible.

Some methods of improving engine break-in performance have involvedapplying sacrificial or abradable coatings to the mating or contactsurfaces of sliding components. These sacrificial coatings are designedto be easily worn away where necessary during initial engine operationso that the contact profiles of the sliding components rapidly conformto each other, leaving little or no clearance therebetween. In order toachieve a desired level of abradability, such coatings are oftentimesmade of polymeric materials and/or dry lubricants which can be readilyworn away and/or transferred from one contact surface to another.However, the abraded portions of these polymeric materials and/or drylubricants may contaminate the operating environment of the engineand/or may mar the contact surfaces of the sliding components.Therefore, there remains a need in the art for an improved method ofenhancing engine break-in performance.

SUMMARY

A piston ring comprising a body having an outer circumferential surfaceis provided. A tribological coating is formed on the outercircumferential surface of the body. The tribological coating includes abase layer and a top layer overlying the base layer. The top layer maycomprise a transition metal nitride-based material, wherein thetransition metal may be selected from the group consisting of titanium(Ti), zirconium (Zr), vanadium (V), niobium (Nb), chromium (Cr),molybdenum (Mo), tungsten (W), and combinations thereof. The top layermay have a relatively high porosity and a relatively low Vickershardness, as compared to the porosity and Vickers hardness of the baselayer.

In one form, the base layer of the tribological coating may comprise atransition metal nitride-based material, and the transition metal may beselected from the group consisting of titanium (Ti), zirconium (Zr),vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo), tungsten(W), and combinations thereof. In another form, the base layer maycomprise a diamond-like carbon (DLC)-based material.

The top layer of the tribological coating may define a cylinder wallengaging surface of the piston ring and may have a contour that exhibitsa plurality of valleys and ridges.

A nitrided layer may be formed at an exterior surface of the body and anintermediate coating may be deposited on the exterior surface of thebody, between the nitrided layer and the tribological coating.

The tribological coating may be deposited on the outer circumferentialsurface of the piston ring by a physical vapor deposition (PVD) process.At least one process parameter of the physical vapor deposition processmay be modified partway through the deposition process such that the toplayer of the tribological coating exhibits a relatively high porosityand a relatively low Vickers hardness, as compared to that of theunderlying base layer.

The piston ring, as described above, may be used in combination with apiston and disposed within a cylinder of a reciprocating internalcombustion engine to form a seal around the piston between thecombustion chamber and the crankcase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevation of a piston and connecting rodassembly for a reciprocating internal combustion engine;

FIG. 2 is a schematic perspective view of a piston ring;

FIG. 3 is a schematic cross-sectional view of the piston ring of FIG. 2taken along line 3-3; and

FIG. 4 is a schematic cross-sectional view of a portion of a contactsurface of a piston ring.

DETAILED DESCRIPTION

The presently disclosed tribological coating may be formed on a contactsurface of a sliding component, such as a piston ring for a piston of areciprocating internal combustion engine. When the tribological coatingis formed on an outer circumferential surface of a piston ring, such asan upper compression ring, the tribological coating may provide thepiston ring with superior short-term and long-term performance, ascompared to prior art piston rings. For example, the presently disclosedtribological coating may allow an effective gas-tight seal to be formedaround a piston in a relatively short amount of time, which may helpstabilize engine performance by reducing combustion gas blow-by andexcess oil consumption. In addition, the tribological coating mayprovide the piston ring with excellent high temperature wear resistance,hardness, and low frictional resistance throughout the life of thepiston ring.

FIG. 1 illustrates a piston and connecting rod assembly 10 for use in acylinder 12 of a reciprocating internal combustion engine (not shown).The assembly 10 has a central longitudinal axis A and comprises a piston14 and a connecting rod 16. When disposed within the cylinder 12, acombustion chamber (not shown) is typically located immediately above anupper surface of the piston 14 and a lubricating oil containingcrankcase (not shown) is typically located below a lower surface of thepiston 14.

The piston 14 has a body including an upper crown 18 and a lower skirt20. A plurality of annular grooves 22 are formed about an outercircumference of the crown 18 of the piston 14 and are sized toaccommodate piston rings, e.g., an upper compression ring 24, a lowercompression ring 26, and an oil control ring 28. Each of the pistonrings 24, 26, 28 has a cylinder wall engaging surface or contact surface34, 36, 38 on an outer circumference thereof that is adapted to contactand slide along an inner wall 40 of the cylinder 12. A pin bore 30 isformed in the skirt 20 of the piston 14 and is sized to receive a pistonpin 32 for connecting the piston 14 to a small end of the connecting rod16.

FIGS. 2 and 3 illustrate a piston ring 110 for a piston of areciprocating internal combustion engine, such as the piston 14illustrated in FIG. 1. The piston ring 110 comprises a split annularbody 112 having an exterior surface that includes an upper surface 114,a lower surface 116, an inner circumferential surface 118, and an outercircumferential surface 120 extending between the upper and lowersurfaces 114, 116. In cross-section, the piston ring 110 illustrated inFIGS. 2 and 3 has a keystone shape, with tapered upper and lowersurfaces 114, 116. However, the piston ring 110 may exhibit variousother cross-sectional shapes, e.g., rectangular. In addition, thecross-sectional profile of the outer circumferential surface 120 of thepiston ring 110 may be generally straight, as illustrated in FIGS. 2 and3, or it may follow an angled or arch-shaped path between the upper andlower surfaces 114, 116. The annular body 112 may be made of cast iron(e.g., gray or nodular cast iron), steel (e.g., stainless steel), or anyother suitable ferrous metal or alloy. The material of the annular body112 may be selected based upon the application and desired performancecharacteristics of the piston ring 110, and/or upon the composition ofany overlying coating layers.

A diffusion nitrided layer 122 may be formed at the exterior surface ofthe annular body 112, although this is not necessarily required. Thenitrided layer 122 may be formed by any known nitriding process. Forexample, the nitrided layer 122 may be formed by heating the annularbody 112 to a suitable temperature and exposing the annular body 112 toa nitrogen-containing gas, e.g., ammonia (NH₃). The nitrided layer 122may extend from the exterior surface of the annular body 112 of thepiston ring 110 at a depth in the range of 10-170 μm. The actual depthof the nitrided layer 122 at the exterior surface of the body 112 may beselected based upon the size of the piston ring 110 and also may beselected to impart certain desirable mechanical and/or physicalproperties to the piston ring 110, including high hardness, wearresistance, scuff resistance, and improved fatigue life. Alternatively,the exterior surface of the annular body 112 may be subjected to adifferent type of thermochemical surface treatment process to produce adifferent type of diffusion layer at the exterior surface of the annularbody 112. Other heat-treatment processes may additionally oralternatively be performed to increase the hardness of select surfaceportions of the annular body 112, including through hardening, hardeningwith isothermal quenching, and/or induction surface hardening. In someinstances, depending on the composition of the annular body 112,additional surface treatment or hardening processes may not beperformed.

Referring now to FIG. 3, in one form, an interlayer or intermediatecoating 124 and a tribological coating 126 are formed on the exteriorsurface of the annular body 112 over the optional nitrided layer 122.The tribological coating 126 may be formed on the exterior surface ofthe annular body 112 over the intermediate coating 124 and/or over oneor more other coating layers already present on the exterior surface ofthe annular body 112. Or the tribological coating 126 may be formeddirectly on the exterior surface of the annular body 112. In such case,the intermediate coating 124 is omitted. Forming the tribologicalcoating 126 directly on the exterior surface of the annular body 112 mayor may not include forming the tribological coating 126 over thenitrided layer 122 or some other type of diffusion layer. This willdepend upon whether the annular body 112 has or has not been subjectedto a nitriding process or some other type of thermochemical surfacetreatment or heat treatment process prior to deposition of thetribological coating 126.

In FIG. 3, the intermediate coating 124 and the tribological coating 126are formed on the outer circumferential surface 120 of the annular body112, with the tribological coating 126 defining a cylinder wall engagingsurface or contact surface 132 of the piston ring 110. In particular,the intermediate coating 124 and the tribological coating 126 are formedon the outer circumferential surface 120 of the annular body 112 suchthat the intermediate coating 124 and the tribological coating 126 bothextend from the upper surface 114 to the lower surface 116 of theannular body 112. In other embodiments, the intermediate coating 124and/or the tribological coating 126 may be additionally or alternativelyformed over one or more other exterior surfaces of the annular body 112,including the upper surface 114, the lower surface 116, and/or the innercircumferential surface 118 of the body 112. In addition, in FIG. 3, theintermediate coating 124 is disposed between the nitrided layer 122 andthe tribological coating 126 on the outer circumferential surface 120 ofthe annular body 112. However, in other embodiments, the intermediatecoating 124 may be omitted and the tribological coating 126 may beformed directly on the outer circumferential surface 120 of the annularbody 112.

The intermediate coating 124 may help improve adhesion of thetribological coating 126 to the exterior surface of the annular body 112and may comprise at least one of chromium (Cr), nickel (Ni), cobalt(Co), titanium (Ti), and vanadium (V). In one form, the intermediatecoating 124 may consist essentially of elemental chromium (Cr). Theintermediate coating 124 may be formed on the exterior surface of theannular body 112 by a thermal spray process (e.g., a flame sprayingprocess, a high velocity oxy-fuel (HVOF) process, or a plasma sprayingprocess), a physical vapor deposition (PVD) process, or by any othersuitable process. A suitable thickness for the intermediate coating 124may be in the range of 1-10 μm, measured in the radial direction of thepiston ring 110. However, in other forms, the thickness of theintermediate coating 124 may be somewhat more or less than this amountdepending on the application method used to form the intermediatecoating 124 on the exterior surface of the annular body 112.

The tribological coating 126 may have a dual layer structure, and mayinclude a relatively hard base layer 128 and a relatively porous toplayer 130. The physical and mechanical properties of the top layer 130and the base layer 128 may be configured to provide the piston ring 110with a combination of excellent short-term and long-term performance.For example, the physical and mechanical properties of the top layer 130may be configured to provide the piston ring 110 with excellentperformance during the initial break-in phase of the piston ring 110,and the base layer 128 may be configured to maintain the hightemperature wear resistance and low frictional resistance of the pistonring 110 for an extended duration. More specifically, it has been foundthat excellent short-term and long-term performance of the piston ring110 may be achieved by decreasing the hardness and increasing theporosity (or decreasing the density) of the top layer 130 of thetribological coating 126 relative to the hardness and porosity (ordensity) of the base layer 128. Increasing the porosity and decreasingthe hardness of the top layer 130 may in turn reduce the internal stressof the top layer 130, relative to the internal stress of the base layer128.

Without intending to be bound by theory, it is believed that therelatively low hardness of the top layer 130 may improve the break-inperformance of the piston ring 110 by allowing the shape of the contactsurface 132 of the piston ring 110 to more readily conform to the shapeof the inner wall 40 of the cylinder 12 during initial engine operationso that the piston ring 110 may be seated to the inner wall 40 of thecylinder 12 in a relatively short amount of time. At the same time, therelatively high hardness of the base layer 128 may provide the pistonring 110 with excellent long-term wear resistance.

The increased porosity (or decreased density) of the top layer 130 ofthe tribological coating 126, relative to the porosity of the base layer128, may provide the contact surface 132 of the piston ring 110 with arelatively rough contour, as shown in FIG. 4.

More specifically, the contact surface 132 of the piston ring 110defined by the top layer 130 of the tribological coating 126 may have acontour that exhibits a plurality of valleys 134 and ridges or plateaus136. Without intending to be bound by theory, it is believed that thevalleys 134 formed along the contact surface 132 of the piston ring 110may allow the contact surface 132 to retain a significant amount ofliquid lubricant (e.g., oil), which may help form a seal and reducefriction between the contact surface 132 of the ring 110 and the innerwall 40 of the cylinder 12 during engine operation. In addition, theretained lubricant on the contact surface 132 of the piston ring 110 mayreduce scuffing between the contact surface 132 of the piston ring 110and the inner wall 40 of the cylinder 12 during initial engineoperation, further enhancing the break-in performance of the piston ring110. At the same time, the relatively high density of the base layer 128may provide the piston ring 110 with a relatively smooth contact surface(not shown) over time, which may provide the piston ring 110 withexcellent long-term frictional behavior.

The ratio of the Vickers hardness of the top layer 130 to the Vickershardness of the base layer 128 may be in the range of 0.5:1 to 0.7:1.The Vickers hardness or microhardness of the base layer 128 and toplayer 130 may be measured according to ASTM E-384 using a 136° pyramidaldiamond indenter on a polished cross section of the piston ring 110. Inone form, the Vickers hardness of the top layer 130 may be greater thanor equal to 800 HV, 900 HV, or 950 HV; less than or equal to 1200 HV,1100 HV, or 1050 HV; or between 800-1200 HV, 900-1100 HV, or 950-1050HV, and the Vickers hardness of the base layer 128 may be greater thanor equal to 1300 HV, 1400 HV, or 1450 HV; less than or equal to 2500 HV,1700 HV, 1600 HV, or 1550 HV; or between 1300-2500 HV, 1300-1700 HV,1400-1600 HV, or 1450-1550 HV. The reduced internal stress of the toplayer 130 may help reduce or eliminate cracking of the tribologicalcoating 126.

In one form, the base layer 128 and the top layer 130 may comprise oneor more Group 4, 5, and/or 6 transition metal nitrides. For example, thebase layer 128 and the top layer 130 may comprise nitrides of titanium(Ti), zirconium (Zr), vanadium (V), niobium (Nb), chromium (Cr),molybdenum (Mo), and/or tungsten (W). In one specific example, both thebase layer 128 and the top layer 130 may comprise a chromium nitride(Cr—N)-based material, as such material may provide the piston ring 110with excellent wear resistance and low frictional resistance between thecontact surface 120 of the piston ring 110 and the inner wall of thecylinder 12. The term “chromium nitride-based material,” as used herein,broadly includes any material or alloy where chromium (Cr) and nitrogen(N) are the predominant constituents of the material, based upon theoverall weight of the material. This may include materials havinggreater than 50 wt % chromium nitride, as well as those having less than50 wt % chromium nitride, so long as chromium (Cr) and nitrogen (N) arethe two largest constituents of the material. In one form, the overallcomposition of the tribological coating 126 may include 40-70 at %chromium (Cr) and 30-60 at % nitrogen (N). In one form, the chromiumnitride-based material may consist essentially of stoichiometric ratiosof chromium nitride (e.g., CrN and/or Cr₂N) and may include a mixture ofCrN and Cr₂N.

The chemical composition of the base layer 128 may be the same ordifferent from that of the top layer 130. For example, in one form, thetop layer 130 may comprise a chromium nitride-based material and thebase layer 128 may comprise an amorphous carbon or diamond-like carbon(DLC)-based material. In such case, the base layer 128 may have aVickers hardness in the range of 1800-2500 HV and the ratio of theVickers hardness of the top layer 130 to the Vickers hardness of thebase layer 128 may be in the range of 0.2:1 to 0.6:1.

The top layer 130 of the tribological coating 126 is distinguishablefrom prior sacrificial or abradable coatings, which are typically madeof polymeric materials and/or dry lubricants and are designed to bereadily worn away and easily transferred from one contact surface toanother. As such, the top layer of the tribological coating 126preferably does not include any polymeric materials or dry lubricants.As used herein, the term “polymeric material” means any material thatcomprises or contains a polymer and may include composite materials thatinclude a combination of a polymer and a non-polymeric material. Theterm “polymer” is used in its broad sense to denote both homopolymersand heteropolymers. Homopolymers are made of a single type of polymer,while heteropolymers (also known as copolymers) are made of two (ormore) different types of monomers. Some examples of polymeric materialsthat are preferably absent from the tribological coating 126 include:acetals; acrylics; acrylonitrile-butadiene-styrene; alkyds; diallylphthalate; epoxy; fluorocarbons; melamine-formaldehyde; nitrile resins;phenolics; polyamides; polyamide-imide; poly(aryl ether); polycarbonate;polyesters; polyimides; polymethylpentene; polyolefins, includingpolyethylene and polypropylene; polyphenylene oxide; polyphenylenesulfide; polyurethanes; silicones; styrenics; sulfones; blockcopolymers; urea-formaldehyde; and vinyls. Some examples of drylubricants that are preferably absent from the tribological coating 126include: graphite, molybdemum disulfide (MoS₂), tungsten disulfide(WS₂), silicates, fluorides, clays, titanium oxides, boron nitride, andtalc.

The tribological coating 126 may have an overall thickness in the rangeof about 5-100 μm, measured in the radial direction of the piston ring110. For example, the overall thickness of the tribological coating 126may be greater than or equal to 20 μm, 30 μm, or 40 μm; less than orequal to 100 μm, 80 μm, or 60 μm; or between 20-100 μm, 30-80 μm, or40-60 μm. The overall thickness of the tribological coating 126 may besomewhat more or less than these amounts depending on the particularapplication of use. The thickness of the top layer 130 may be less thanthat of the base layer 128 and may account for approximately 5% to 50%of the overall thickness of the tribological coating 126, orapproximately 5% to 30% of the overall thickness of the tribologicalcoating 126. The thickness of the top layer 130 may be greater than orequal to 5 μm, 8 μm, or 11 μm; less than or equal to 25 μm, 20 μm, or 16μm; or between 5-25 μm, 8-20 μm, or 11-16 μm, and the thickness of thebase layer 128 may be greater than or equal to 25 μm, 30 μm, or 32 μm;less than or equal to 50 μm, 40 μm, or 35 μm; or between 25-50 μm, 30-40μm, or 32-35 μm. The ratio of the thickness of the top layer 130 to thethickness of the base layer 128 may vary depending on the application ofthe piston ring 110 and the operating parameters of the engine.

The tribological coating 126 may be formed on the exterior surface ofthe annular body 112 via any suitable deposition technique. For example,the tribological coating 126 may be formed on the exterior surface ofthe annular body 112 by physical vapor deposition (PVD) (e.g., cathodicarc or sputtering), chemical vapor deposition, vacuum deposition, orsputter deposition.

In one form, the tribological coating 126 may be formed on the exteriorsurface of the annular body 112 by a cathodic arc physical vapordeposition process that includes: (i) positioning the annular body 112in a deposition chamber including an anode and at least one solidcathode source material; (ii) evacuating the deposition chamber; (iii)introducing a process gas into the deposition chamber; (iv) striking andmaintaining an electric arc between a surface of the cathode sourcematerial and the anode such that portions of the cathode source materialare vaporized; and (v) depositing the vaporized cathode source materialon the exterior surface of the annular body 112.

The solid cathode source material may comprise pure elemental chromium(Cr) and the process gas may comprise a reactive nitrogen-containinggas. In such case, the vaporized chromium may react with nitrogen gas inthe deposition chamber to form compounds of chromium nitride, which maybe deposited on the exterior surface of the annular body 112 to form thetribological coating 126. The operating pressure within the depositionchamber during the deposition process may be in the range of 0-0.1 mbarand may be controlled by suitable adjustment to the flow rate of aninert gas (e.g., argon (Ar)) and/or the flow rate of nitrogen gas thatis introduced into the deposition chamber as a constituent of thereactive nitrogen-containing gas. A negative voltage in the range of 0volts to −150 volts (referred to as a bias voltage) may be applied tothe annular body 112 during the deposition process to help acceleratethe positively charged ions from the solid cathode source material tothe exterior surface of the annular body 112. The duration of thedeposition process may be controlled or adjusted to achieve atribological coating 126 having a desired thickness. In one form, thedeposition process may be performed at a deposition rate of 2-4 μm perhour and for a duration of 6-24 hours.

Various process parameters may be varied or modified partway through thedeposition process to achieve the dual layer structure of thetribological coating 126. For example, the base layer 128 may be formedduring a first stage of the deposition process. Then, after depositionof the base layer 128, certain process parameters may be changed toinitiate a second stage of the deposition process wherein the top layer130 is formed directly on and over the base layer 128. Deposition of thetop layer 130 and the base layer 128 of the tribological coating 126 maybe performed by modifying certain process parameters partway through thecathodic arc physical deposition process, without having to purchaseadditional manufacturing equipment and without having to extend theduration of the overall piston ring 110 manufacturing process. In oneform, the first stage of the deposition process may be performed at afirst operating pressure and the second stage of the deposition processmay be performed at a second operating pressure greater than the firstoperating pressure. For example, the operating pressure of the nitrogenmay be adjusted and increased during the deposition process to achieve adesire characteristic in both the base and top layers 128, 130. In onespecific example, the operating pressure during the first stage of thedeposition process may be about 0.03 mbar, and the operating pressureduring the second stage of the deposition process may be about 0.05mbar. Increasing the operating pressure during the second stage of thedeposition process may increase the porosity and also may decrease thehardness of the chromium nitride material that is being deposited on theexternal surface of the annular body 112. Increasing the operatingpressure during the second stage of the deposition process may result inthe emission of relatively large droplets from the cathode sourcematerial, which may be deposited on the exterior surface of the annularbody 112 over the base layer 128 and may modify the size of theparticles or grains formed within the top layer 130, providing acombined characteristic of lower hardness and increased porosity.

A bias voltage may be applied to the annular body 112 during the firststage of the deposition process, but may not be applied to the annularbody 112 during the second stage of the deposition process. In onespecific example, a bias voltage of about 50 volts may be applied to theannular body 112 during the first stage of the deposition process.Applying a bias voltage to the annular body 112 during the first stageof the deposition process (but not the second stage) may result in theformation of a relatively hard base layer 128 and a relatively soft toplayer 130. In another example, a bias voltage may be applied to theannular body 112 during both the first and second stages of thedeposition process. In such case, the bias voltage applied to theannular body 112 during the first stage of the deposition process may bedifferent from the bias voltage applied to the annular body 112 duringthe second stage of the deposition process. Changing the bias voltagebetween first and second stages of the deposition process may allow thetop layer 130 to be formed with lower hardness and increased porosity,as compared to that of the base layer 128.

In addition to operating pressure and bias voltage, one or more otherprocess parameters may be modified or changed partway through thedeposition process to differentiate the chemical and/or mechanicalproperties of the top layer 130 and the base layer 128 and therebyimprove the short-term and/or long-term performance of the piston ring110. Some examples of additional process parameters that may be modifiedduring the deposition may include any of the various process parametersincluding arc current, process temperature, and process time.

After deposition of the tribological coating 126, the surface of the toplayer 130 may have a contour that exhibits a plurality of valleys andpeaks. In such case, the outer circumferential surface of the pistonring 110 may be ground and lapped to transform the peaks into relativelyflat ridges or plateaus, which may help prevent scuffing of the innerwall of the cylinder 12 during the running-in phase.

FIG. 4 is a schematic cross-sectional view of a portion of the pistonring 110 illustrating the morphology of the diffusion nitrided layer122, the intermediate coating 124, and the tribological coating 126formed on and over the outer circumferential surface 120 of the annularbody 112 of the piston ring 110 at 500 times magnification. Thetribological coating illustrated in FIG. 4 may be produced using acathodic arc physical vapor deposition process. As shown, a gradualtransition in the microstructure of the tribological coating 126 maytake place between the base layer 128 and the overlying top layer 130 asa result of a step-wise modification of the deposition processparameters partway through the deposition process.

It is to be understood that the foregoing is a description of one ormore preferred exemplary embodiments of the invention. The invention isnot limited to the particular embodiment(s) disclosed herein, but ratheris defined solely by the claims below. Furthermore, the statementscontained in the foregoing description relate to particular embodimentsand are not to be construed as limitations on the scope of the inventionor on the definition of terms used in the claims, except where a term orphrase is expressly defined above. Various other embodiments and variouschanges and modifications to the disclosed embodiment(s) will becomeapparent to those skilled in the art. All such other embodiments,changes, and modifications are intended to come within the scope of theappended claims.

As used in this specification and claims, the terms “for example,”“e.g.,” “for instance,” “such as,” and “like,” and the verbs“comprising,” “having,” “including,” and their other verb forms, whenused in conjunction with a listing of one or more components or otheritems, are each to be construed as open-ended, meaning that the listingis not to be considered as excluding other, additional components oritems. Other terms are to be construed using their broadest reasonablemeaning unless they are used in a context that requires a differentinterpretation.

The invention claimed is:
 1. A piston ring comprising: a body made of aferrous material and having an outer circumferential surface; adiffusion nitride layer formed on the outer circumferential surface ofthe body and extending at a radial depth therein; an intermediatecoating directly over the diffusion nitride layer; and a tribologicalcoating directly overlying the intermediate coating that includes a baselayer and a top layer overlying the base layer, wherein both the baselayer and the top layer each include a transition metal nitride, thetransition metal of the top layer being selected from the groupconsisting of titanium (Ti), zirconium (Zr), vanadium (V), niobium (Nb),chromium (Cr), molybdenum (Mo), tungsten (W), and combinations thereof,and wherein the top layer has a relatively high porosity and arelatively low Vickers hardness, as compared respectively to a porosityand a Vickers hardness of the base layer.
 2. The piston ring set forthin claim 1 wherein a ratio of the Vickers hardness of the top layer tothe Vickers hardness of the base layer is in the range of 0.2:1 to0.7:1.
 3. The piston ring set forth in claim 1 wherein the top layer hasthe Vickers hardness in the range of 800-1200 HV and the base layer hasthe Vickers hardness in the range of 1300-2500 HV.
 4. The piston ringset forth in claim 1 wherein the transition metal of the base layer isselected from the group consisting of titanium (Ti), zirconium (Zr),vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo), tungsten(W), and combinations thereof, and wherein the base layer has a Vickershardness in the range of 1300-1700 HV.
 5. The piston ring set forth inclaim 4 wherein a ratio of the Vickers hardness of the top layer to theVickers hardness of the base layer is in the range of 0.5:1 to 0.7:1. 6.The piston ring set forth in claim 1 wherein the top layer comprises achromium based material.
 7. The piston ring set forth in claim 6 whereinthe base layer comprises a chromium based material.
 8. The piston ringset forth in claim 7 wherein the chromium based material comprises 40-70at % chromium (Cr) and 30-60 at % nitrogen (N).
 9. The piston ring setforth in claim 1 wherein the top layer has a thickness in the range of5-25 μm and the base layer has a thickness in the range of 25-50 μm, andwherein the thickness of the top layer is less than the thickness of thebase layer.
 10. The piston ring set forth in claim 1 wherein thetribological coating has a thickness in the range of 5-100 μm.
 11. Thepiston ring set forth in claim 1, wherein the base layer and the toplayer are of the same chemical composition.
 12. The piston ring setforth in claim 1, wherein the base layer and the top layer are ofdifferent chemical compositions.